Recovery from extended day and night schedules Merkus, S.L.
2017
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Merkus, S. L. (2017). Recovery from extended day and night schedules.
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C h a p t e r 4
The effects of shift work on body weight change - a systematic review of longitudinal studies
alwin van Drongelen Cécile r.L. Boot Suzanne L. Merkus tjabe Smid
allard J. van der Beek
Scandinavian Journal of Work Environment and Health 2011;37(4):263-75
Abstract
Objective: This systematic review aims to summarise the available evidence and thereby elucidate the effects of shift work which includes night work, on body weight change.
Methods: A systematic search strategy on longitudinal studies was performed. Articles were included following strict inclusion criteria, methodological quality was assessed by a stan- dardised quality checklist. The results were summarised using a levels of evidence synthesis.
Results: The search strategy resulted in eight articles that met the inclusion criteria. Five of them were considered as high quality and three as low quality studies. Seven studies pre- sented crude results for an association between shift work exposure and change in body weight; five high quality and two low quality studies. There was strong evidence for a crude relationship between shift work and body weight increase. Five studies presented weight related outcomes adjusted for potentially relevant confounders (age, gender, bodyweight at baseline and physical activity). Two studies found a significant difference between groups in the same direction. Consequently, the evidence for a for these confounders adjusted relation- ship between shift work exposure and body weight was considered to be insufficient.
Conclusion: Strong evidence for a crude association between shift work exposure and body weight increase was found. In order to further clarify the underlying mechanisms, more and better high quality studies about this subject are necessary.
4
Introduction
Due to the 24/7 economy, the number of jobs including shift work and irregular working hours has increased substantially during the last decades. Up to an estimated 20% of the European workers are thought to be exposed to shift work schedules that include time spent working at night (1). A recent report of the European Working Conditions Survey (2) indicates that this proportion has stabilised during the last 10 years. In the Netherlands, more than 20%
of the working population reports that they work in shifts while approximately 50% reports occasional work during the evening or night (3). An overview article by Costa (4) stated that only 24% of the workforce still has a regular daytime, Monday to Friday working week.
A large amount of research has been published concerning the health effects of shift work.
This research shows that, although the pathways are unclear, shift schedules including nightly hours are probably related to gastrointestinal and cardiovascular diseases (1,5-9). Further, this kind of work is thought to be associated with an increased risk for several types of cancer (10-13).
Shift work including night work has also been found to be associated with diabetes and metabolic disturbances, but the evidence is not conclusive (6,14,15). However, recent longi- tudinal studies showed significantly increased risks for the metabolic syndrome in shift and night health care workers (16,17). Still, the underlying mechanisms for the onset of metabolic health problems as a result of shift work are not clear yet. Possible mediators are reduced sleep and physical activity, changed eating habits and patterns, and an altered circadian rhythm (18-20). Weight gain, overweight or obesity might be the mediating factor between shift work and metabolic disturbances, which can eventually result in diabetes or cardiovascular diseases (1,5).
Overweight and obesity are major health care problems for current society. As an indirect result of the population getting heavier, health care costs are thought to be increasing. Estima- tions vary from 1 to 7% of the total health care cost in the developed world, and the amount of indirect costs due to loss of productivity and disability pensions might be even higher (21). It has been shown that overweight and obesity lead to an elevated risk of some forms of cancer, cardiovascular and digestive diseases, diabetes mellitus, sleep apnoea, and osteoarthritis (22,23). In a large cohort study for example, Colditz et al. (24) showed that body mass index (BMI) is a main predictor for the onset of diabetes. In a recent review it was found that due to obesity, the all cause mortality is increased by approximately 20% (25).
In recent years, several studies have investigated the association between shift work and weight gain, overweight or obesity (26-31). Several of them demonstrated a significant rela-
tion between working in shifts and body weight or obesity (26-29), while some others did not (30,31). Most of these studies, however, had a cross-sectional design and thereby failed to determine a clear causal relationship between shift work and body weight change. Lon- gitudinal research would be able to give more insight in the working mechanisms of this relationship and results could be used in efficient worksite prevention. This could eventually lead to a reduced risk for various kinds of diseases and a smaller burden on health care costs.
Up to date, no systematic review has been published about the effects of shift work on body weight. This review aims to summarise the available evidence and thereby elucidate the relationship between exposure to shift work, which includes night work, and body weight change. A systematic literature search, selection and quality assessment has been performed.
In order to be able to find indications for causality, only longitudinal studies are included.
Method
Search StrategyThe literature search aimed to identify relevant peer-reviewed studies providing information about change in body weight as a consequence of shift work.
The review was based on publications retrieved by a computerised search of the following databases: Medline, Embase, Cochrane library, and PsycINFO. The databases were searched for published articles up to June 18, 2010. The search terms included were: work schedule tolerance (Mesh), shift work, night work, irregular working hours, body weights and measures (Mesh), body weight changes (Mesh), BMI and body mass. After inclusion of the articles based on the selection criteria below, references were checked for additional articles.
The full Medline search strategy can be seen in Appendix 1, the search strategies for the other databases were based on this strategy.
Selection criteria
The retrieved abstracts were checked on the following criteria:
1. The study was a full text, peer reviewed article in written English, Dutch, German, French, Spanish or Italian.
2. The study compared a group of shift workers with a control group of day workers. Working at night (12.00 am-6.00 am) was part of the shift work exposure.
3. The study had a longitudinal design; either prospective or retrospective.
4 4. The outcome included: body weight, BMI, waist to hip ratio or waist circumference. At least
two measurements of the outcome were provided or the difference between outcome measurements was presented.
5. The study presented the association between shift work and day work and an outcome related to body weight, or this association could be calculated by the authors.
Two reviewers (AvD and CB) read all the abstracts independently. If no abstract was available or if it was not clear whether the article should be included based on the title or abstract, the full text article was read. Articles were included if they met al.l five inclusion criteria. If consen- sus between the two reviewers could not be reached, a third reviewer made a final decision.
Quality assessment
Differences in methodological quality across studies indicate that the results of some studies are more likely to be affected by bias than others, making it important to take the quality of a study into account. Two reviewers (AvD and SM) independently assessed the quality of each included study.
A standardised checklist of predefined criteria was used, which was a modified version of the checklists by Hayden et al. (32) and Van der Windt et al. (33) (Table 1). Each item was scored as positive (+) or negative (-, potential bias). If the paper provided insufficient information on the specific item, the item was scored with a question mark (?, don’t know). If an item was not applicable, it was scored as not applicable (NA).
Disagreements between the reviewers on individual items were identified and solved during a consensus meeting. Subsequently, the first author of each included article with a quality item that had scored a question mark was contacted in order to provide the author an opportunity to clarify that quality item of their article.
Eventually, for each study, a total quality score was calculated by counting the number of items that were rated positively. This number was divided by the total number of applicable items of the study. Based on this total score, a study was classified as either a high or low quality study. A study was classified as high quality if a study scored positively on at least 51%
of the applicable items in the quality assessment list; otherwise, a study was classified as low quality. High quality studies were considered to have an overall low risk of bias, while the low quality studies have a high risk of bias. This is in accordance with previously published systematic reviews (21,33-35).
Data extraction
Details about the following elements were extracted and tabulated from the publications:
study population, sample size, response rate, study design, follow-up duration, exposure,
controlled confounders, outcome measurement, results without adjustment for confounders, results with adjustment for confounders, and overall conclusion.
The associations between exposure to shift work and body weight outcome were presented as the mean differences in outcome between baseline and follow-up. If possible, the 95%
confidence intervals (CI) were calculated and presented as well.
The first author of the included article was contacted to provide additional information if calculation of the adjusted mean difference or p-value was not possible.
Table 1: Checklist of methodological quality (based on hayden et al. (32) and Van der Windt et al. (33)) Study objective
1 positive if a specific, clearly stated objective is described.
Study population
2 positive if the main features of the study population are clearly described (sampling frame and distribution of the study population by age and gender).
3 positive if the inclusion and exclusion criteria are clearly described.
4 positive if the participation rate is ≥ 80%, or if it is lower, the non-response is not selective (data presented).
5 positive if the response rate (proportion completing the study and providing outcome data) is ≥ 80% or if the non-response is not selective (data presented).
6 positive if information (potential confounders and outcomes) is presented about the population lost to follow-up.
Exposure assessment
7 positive if the characteristics of shift work are clearly described (type of work and shift schedules).
8 positive if data are collected and presented about occupational exposure in the past and during the follow- up period.
Confounder assessment
9 positive if the most important confounders (age, gender, body weight at baseline, physical activity) are measured and used in the analysis for body weight outcome.
10 positive if confounders are measured the same for all participants using standardised methods of acceptable quality.
Outcome assessment
11 positive if body weight is measured the same for all participants using a standardised method of physical examination.
12 positive if data were collected for ≥ 1 year.
Analysis and data presentation
13 positive if the appropriate statistical model is used to evaluate the data.
14 positive if mean differences in body weight outcome are presented or can be calculated (including 95% CI).
4 Analysis
Results of the studies were analysed and where possible, a statistical meta-analysis was per- formed. To gain insight in factors interfering with the relationship between shift work and body weight change, crude and adjusted results were analysed separately.
To summarise the results and thereby draw conclusions about the relationship, a levels of evidence synthesis was used. This synthesis took into account the methodological quality and the outcomes of the selected studies. It was applied to both the crude and adjusted weight related outcomes of the studies.
The three levels used were based on Hoogendoorn et al. (34) and Sacket et al. (36):
– Strong evidence: consistent findings in multiple high quality cohort studies.
– Moderate evidence: consistent findings in one high quality cohort study and in one or more low quality cohort studies.
– Insufficient evidence: only one study available or inconsistent findings in multiple cohort studies.
Findings were considered consistent if at least 75% of the selected cohort studies showed significant (p<0.05) results in the same direction.
Results
Study selection
The results of the selection procedure are presented in Figure 1. The search strategy resulted in 1047 citations (571 PubMed, 291 Embase, 160 Cochrane, and 25 PsychINFO). After excluding the doublings, 839 titles and abstracts were examined. Out of these, 65 full texts were selected for further investigation. Seven articles met all inclusion criteria. The references of these articles were checked and this resulted in one additional article. The percentage agreement between the two reviewers was 95%. Initial disagreement about three of the 65 studies (resulting in a Cohen’s κ of 0.86) was resolved in a consensus meeting.
The most frequent reason for exclusion was the fact that the studies did not measure a change in body weight or another appropriate outcome. Many studies did measure body weight or BMI multiple times, but only used the baseline measurement as a confounder or covariate for their specific outcome. Therefore, change in weight outcome was not presented or it could not be calculated.
Methodological quality assessment
The outcome of the quality assessment is presented in Table 2. The scoring of the quality items by the two reviewers resulted in an initial agreement of 75% (Cohen’s κ of 0.56). All disagree- ments were resolved in a consensus meeting between the two reviewers.
Articles screened on title and abstract: 839
Articles excluded due to doublings: 208
Articles excluded based on abstract: 774 Full text articles
retreived: 65
Articles meeting the inclusion criteria: 7
Excluded articles: 58 Study type: 2 Study population: 15
Study design: 10 Outcome: 31
Articles included: 8 Articles retreived in
references: 1
Number of potentially relevant articles found in the search strategy:
1,047
Figure 1. result of the selection procedure
Table 2: results of the methodological quality assessment
Study Methodological items Total
score Total
%
Quality
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Suwazono et al. (37) + + + + +* - + - + ? + + + +* 11/14 79% high
Morikawa et al. (38) + ? - ? + - + + + + - + + + 9/14 64% high
Niedhammer et al. (39) + + - - - + + + + - + + + ? 9/14 64% high
Geliebter et al. (43) + + + + Na Na + - - ? - - + + 7/12 58% high
van amelsvoort et al. (40) + + + - - - + - - + + + + - 8/14 57% high
Copertaro et al. (41) + + + ? ? ? + + - ? ? + + - 7/14 50% low
hannerz et al. (42) - + - + - - - - + + - + + + 7/14 50% low
romon et al. (44) + + - - Na Na + - - ? - ? + - 4/12 33% low
4
Table 3: Study characteristics StudyQual. scoreStudy populationSample sizeResponse rateStudy designFollow up timeExposureControlled confoundersOutcome measurement Suwazono et al. (37)79%Japanese male workers at a steel company n=7254 2926 alternating shift 4328 day shift none reportedprospective study on the effect of shift work on weight gain 14 yearsDay shifts: no hours reported alternating: shifts starting at either 7am, 15pm or 23pm.
age, BMI measured during the study, drinking, smoking and regular exercise
BMI obtained by annual health examination was calculated into the percentage BMI increase relative to BMI at job entry. Morikawa et al. (38)64%Japanese male blue collar workers at a sash and zipper factory
n=1144 712 dayworkers 434 shiftworkers
84%prospective study on the effects of shiftwork on changes in metabolic disturbance parameters.
10 yearsn (two measurements)Day work: no specifications presented Shift work: various systems age, BMI, smoking, alcohol consumption and leisure time physical activity at baseline
Change in BMI, calculated after obtaining weight and height at the beginning and end of the study Niedhammer et al. (39)64%French female nurses1985: n=363. 215 day work, 148 night work 1990:n=305. 216 day work, 9 night work
1985: 89% 1990: 78%prospective study on the relationship between night work, overweight and weight gain.
5 years (two measurements) 5 years (two measurements)
Day work: permanent or alternating day and evening work Night work: permanent night work or alternating morning, evening and night work age, BMI at baseline, births during the 5year period, smoking and sports activities.
Weight gain measured by a physician each 5 years. (plus a correlation coefficient between exposure to night work and weight gain.
Study characteristics (continued) Qual. scoreStudy populationSample sizeResponse rateStudy designFollow up timeExposureControlled confoundersOutcome measurement er et al. 58%US male and female nurses, nurse’s aids and security personell n=85 49 late shifts 36 day shifts none reportedretrospective measurement of weight gain during day and night work NoneDay shift: 8am to 4pm Late shift: 4pm to 12am or 12am to 8am
age, years on the shift, smoking statusSelf-reported weight change since the start of their current job type oort 57%Mix of nurses, workers at a incinerator plant and other employees
n=264 105 dayworkers 159 shiftworkers
65%prospective study on the impact of one year shift work on cardiovascular disease risk factors 1 yearDay work: no specifications presented Shift work: working in an alternating work schedule, including nights age, gender, BMI at baseline, physical sporting activity and physical activity during leisure time. Data obtained after contacting the first author.
Measured change in BMI and waist circumference o et 50%Italian female and male nursesn=58 30 rotating shifts 28 daytime shifts
none reportedprospective measurement of predictors for cardiovascular disease 18 months (four measurements)Day shifts: 8am-14pm or 14pm-22pm rotating shifts: Starting at 6am, 14pm or 22pm.
N/aWaist circumference (cm) measured by medics.
4
Table 3: Study characteristics (continued) StudyQual. scoreStudy populationSample sizeResponse rateStudy designFollow up timeExposureControlled confoundersOutcome measurement hannerz et al. (42)50%random male sample from the Central population register of Denmark n=1980 379 irregular working 1601regular working 76%prospective study on work factors related with changes in BMI.
5 years (two measurements)N/aage, cohabiting, smoking, baseline bmi, work hours/ week, cold work environment, hot work environment, physical activity at work, decision authority, psychological demands, possibilities to communicate with colleagues, conflicts at work, job insecurity.
Self-reported height and weight was calculated into ΔBMI romon et al. (44)33%French male factory workersn=84 27 3-rotating shift 47 5-rotating shift 20 day shiftnone reportedretrospective dietary survey of annual weight gain
NoneDay shifts: permanent day work between 7.30am and 17.30pm rotation shifts: shift work starting at 5am, 13pm or 21pm.
None. Groups were matched on age, socio-professionel level and seniority annual weight gain in kg/ year since the start of the shift type. Obtained by occupation health service records.
Two out of six authors replied to our invitation to clarify the methodological quality of their article. As a result, two unclear items (?) were changed into positive (+). Eventually, five out of eight articles were considered high quality studies.
Study characteristics
Large differences existed between the studies. Especially the kind of shift work exposure, population, sample size, and duration of follow-up differed substantially. For this reason no overall meta-analysis was performed on the outcome data. The characteristics of the eight studies included in this review are presented in Table 3.
Study design
Six studies had a prospective design (37-42) and two retrospective studies were included (43,44). The follow-up periods of the prospective studies ranged from 12 months (40) up to 14 years (37). In the retrospective studies, data were obtained from the start of the participants’
employment in the job (43,44).
Population
Four studies reported findings from a population of nurses (39-41,43). Besides nurses, Geli- ebter et al. (43) included security personnel as well, while van Amelsvoort et al. (40) included a mixture of nurses and factory personnel. Three studies reported about male factory personnel only (37,38,44). Hannerz et al. (42) used a random nationwide sample. In this study, a male sample from the Central Population Register of Denmark was studied and participants with poor self-rated health at baseline were excluded.
The number of participants in the selected studies ranged from 55 (41) to 7,254 (37).
Suwazono et al. (37) did not report a dropout rate for the 14 years of follow-up, while Cop- ertaro et al. (41) did not report any loss to follow-up during 18 months. Van Amelsvoort et al.
(40) retained 70% of their initial sample after 12 months. Morikawa et al. (38) lost 16% during 10 years. Hannerz et al. (42) lost 18% to follow-up in five years, while Niedhammer et al. (39) lost 5% in 1985, and 16% in 1990.
Exposure
The authors of the selected studies used different definitions for shift, night and day work. A summary of the exposure characteristics is presented in Table 4.
The working hours of the experimental groups in some studies consisted of three different shifts (morning, evening, and night) with a continuous (covering the whole week) rotating
4 systems (38), participants exposed to both continuous and non-continuous (covering only
week days) systems (44), and to counterclockwise shift systems (38,40). Further, some studies included nurses with permanent night (39,43) or evening (43) work. The exposure in these two latter studies shows the profound differences between studies because another study excluded all participants with fixed, permanent night work (37).
Table 4: exposure characteristics Study Quality
score
Shift work exposure Day work exposure
shift system schedule continuity rotation shifts working time Suwazono et
al. (37)
79% four team/
three shift system
five days on, two off, five evenings on, one off, five nights on, two off
continuous clockwise no information presented
Morikawa et al. (38)
64% two-shift or three-shift system
five on, two off and three or four on, one off (three shift)
continuous and non- continuous
counter clockwise (three shift)
no information presented
Niedhammer et al. (39)
64% three shift or permanent night
several kinds no information presented
clockwise permanent day or morning and evening shifts
9am-17pm or 6am- 14pm and 13 pm-21pm
Geliebter et al. (43)
58% permanent night or evening shift
fixed no
information presented
none none 8am-4pm
van amelsvoort et al. (40)
57% all kinds of exposure: 32% fast clockwise schedule (at most three consecutive night shifts) 17% fast counter clockwise schedule (at most three consecutive night shifts) 35% medium counter clockwise schedule (at most five consecutive night shifts) 15% irregular schedule
no information presented
Copertaro et al. (41)
50% three shift system
one morning, one evening, one night, two off
continuous clockwise morning and afternoon
8am-14pm and 14pm- 22pm
hannerz et al.
(42)
50% all kinds of irregular working hours permanent day duty
romon et al.
(44)
33% three shift system
five on, two off (slow) or three on, one off (fast)
continuous and non- continuous
no information presented
none 8hours
between 7.30am- 17.30pm
Differences in day work definitions were present as well. The control group of three studies worked between approximately 7.00 am and 5.30 pm (42-44). Other studies did not give any specifications of the day working hours (37,38,40), while participants in the control group of some other studies worked in morning or evening shifts themselves (Table 4) (39,41).
Outcome measurement
Different outcome measures for body weight change were used. Several studies measured body weight and calculated it in kilograms of body weight change (39,43,44). Others obtained body weight and body height and converted this into BMI and BMI change (37,38,42). Cop- ertaro et al. (41) reported a change in mean waist circumference. Van Amelsvoort et al. (40) was the only study that, in addition to change in body weight and BMI, presented waist to hip ratio as well.
Secondly, there was a difference in the way the outcome was measured. In six out of eight studies, qualified people (e.g. physicians) measured the body weight related outcomes (37- 41,44). Two studies based their outcomes on self-reported body weight changes provided by the participants (42,43).
Confounding
The different studies measured a wide variety of potential confounders. These confounders however, were not always used in the analyses to adjust the body weight related outcomes (Table 3).
Outcomes
The unadjusted and adjusted results on body weight related outcome of the eight included ar- ticles are presented in Table 5. Two articles provided sufficient information about the outcome (42,44), while mean values could be calculated in two other studies (38,43). Four first authors were contacted by email and asked to provide additional data for adjusted mean difference or p-value calculation. One author did not respond (41), another author did, but could not reproduce the data (39). Van Amelsvoort (40) provided data about the adjusted difference in several outcomes between groups. Suwazono (37) provided data about the mean percentual BMI change for both groups. However, the author reported that due to job schedule type changes, this mean change was only available for a one year period from baseline (while the follow-up period of the study was 14 years).
Associations between shift work and body weight change, crude results
Seven out of the eight studies presented crude, non-adjusted, outcomes of type of work and body weight change: five high and two low quality studies. The five high quality studies (37-
4
Table 5: Study results StudyQual. scoreCrudeAdjusedConclusion Suwazono et al. (37)79%provided data by the first author, %BMI change after one year from baseline: shift ΔBMI 0,56% (0,44% - 0,69%) 95% CI day ΔBMI 0,31% (0,21% - 0,42%) 95% CI p=0.004 provided data by the first author, %BMI change after one year from baseline: shift ΔBMI 0,63% (0,44% - 0,82%) 95% CI day ΔBMI 0,40% (CI 0,24% - 0,56%) 95% CI p=0.002
Shiftworkers had a significant higher increase in BMI after one year from baseline compared to dayworkers. No differences in results between adjusted and non-adjusted analysis. Morikawa et al. (38)64%Mean BMI change for shift-shift workers was 0.88(se 0.07) Mean BMI change for day-day workers was 0.63(se 0.06) p= 0.002, calculated mean difference 0.25 (se 0.08)
Mean BMI change for shift-shift workers was 0.89(se 0.07) Mean BMI change for day-day workers was 0.62(se 0.06) p= 0.001, calculated mean difference 0.83 (se 0.25) there was a significant higher BMI change in shift-shift workers after 10 years of follow-up compared to the day-day workers. Niedhammer et al. (39)64%1985. Mean weight change for current night workers was 0.3kg, for current day workers 0.6kg. NS.
1985. Correlation coefficient: -0.0 (NS).1985. No significant difference in weight gain between groups after 5 years. Both crude and adjusted. 1990. Mean weight change for night workers was 2.2kg, for day workers 1.3kg. Significant difference but no p-value reported.
1990. Correlation coefficient: 0.8 (0.08) NS.1990. a significant crude difference in weight gain between groups after 5 years. Geliebter et al. (43)58%Late shift weight gain since start of current job type = 4.3kg Day shift weight gain = 0.9kg p=0.02, calculated mean difference is 3.4 (se 1.43) Late shift weight gain = 4.4kg Day shift weight gain = 0.7kg p=0.008, calculated mean difference is 3.7 (se 1.36) Significant more reported weight change in late shift workers compared to day shift workers. Both crude and adjusted.
Study results (continued) StudyQual. scoreCrudeAdjusedConclusion oort et 57%Mean weight change shift work: -0.98 kg Mean weight change day work: 0.43 kg p=0.003. Calculated Mean difference 1,41 kg (se 0.47) adjusted data below provided by the first author: Mean weight change shift work: -1.02 kg Mean weight change day work: 0.28 kg p=0.007. Mean difference 1,30 kg (se 0.48) Significant crude difference between groups in weight and BMI change. Shift workers lost weight and BMI compared to day workers who gained weight and BMI. Mean BMI change shift work: -0.31 Mean BMI change day work: 0.13 p=0.004. Calculated mean difference 0.44 (se 0.15)
Mean BMI change shift work: -0.33 Mean BMI change day work: 0.07 p=0.011. Mean difference 0.40 (se 0.16) Mean waist to hip ratio change shift work: -0.0093 Mean waist to hip change day work: -0.0052 p=0.3. Calculated mean difference 0.0041 (se 0.0042)
Mean waist to hip ratio change shift work: -0.0102 Mean waist to hip change day work: -0.0053 p=0.256. Mean difference 0.0049 (se 0.0043) o et al. (41)50%rotating nurses’ circumference changed from 95.8 cm (sd 9.1) to 95.7 cm(sd 9.2). Mean difference of -0.1 in 18 months Daytime nurses’ circumference changed from 90.2(sd 10.2) to 90.4 (sd 9.9), mean difference of +0,2 in 18 months. No p-value reported
N/aalthough there was no p-value presented, no crude difference was found between groups. z et al. (42)50%N/aIrregular working hours lead to a ΔBMI of -0.05 (-.024, 0.15) 95% CI compared to the group without irregular working hours p=0.6382
No adjusted difference found between groups . (44)33%annual weight gain in: 3 day rotating shifts: 0.73kg (sd 0.30). 5 day rotating shifts: 0.89kg (sd 0.28). Day shifts: 1.02kg (sd 0.54) NS
N/aNo significant crude difference in annual weight gain between groups.
4 et al. (37,38) found more body weight related change for shift workers in a male Japanese
population, while Niedhammer et al. (39) and Geliebter et al. (43) reported similar results for male and female nurses in France and the US, respectively. Van Amelsvoort et al. (40) however, found significant results in the other direction. The two low quality studies (41,44) did not find any significant difference in body weight related outcome between groups.
Because multiple high quality studies provided consistent (four out of five studies; 80%) findings, it is concluded that there is strong evidence for a crude relationship between shift work and body weight change.
Associations between shift work and body weight change, adjusted for potentially relevant confounders
Six out of eight articles presented body weight related outcomes adjusted for potential confounders, five high quality studies and one low quality study. The high quality study of Geliebter et al. (43) found a significant difference in body weight change between late shift and day shift nurses, while adjusting for age, years in shift work, and smoking status. The po- tentially relevant confounders gender, body weight at baseline, and physical activity were not taken into account and therefore, the study was not considered in the analysis.
The other studies showed varying results. Hannerz et al. (42) found no significant difference in their Danish male sample. Niedhammer et al. (39) did not find a significant effect of night work on body weight change in a nurses population. The two high quality studies among male Japanese workers did find a significant effect. Morikawa et al. (38) found a difference in BMI increase between shift workers and day workers after a follow-up of 10 years. Suwazono et al. (37) showed a significantly higher BMI increase after one year for shift workers compared to day workers. Again, the study of van Amelsvoort et al. (40) found results in the other direction.
The shift workers showed a loss in weight outcome after adjustment, which was significantly different from the day workers.
Because two out of four high quality studies (50%) found a significant difference in the same direction, the results are considered to be inconsistent. Subsequently, if the low quality study of Hannerz et al. (42) is included, only 40% (two out of five) of the studies show results in the same direction. Therefore moderate evidence was rejected as well, and it is concluded that there is insufficient evidence for an adjusted relationship between shift work and body weight change.
Discussion
The present paper is the first systematic review of the scientific literature to investigate the association between exposure to shift work which includes night work, and body weight change. The included studies show that there is strong evidence for a crude relationship between longitudinal exposure to shift work and body weight outcomes. Additionally, it is concluded that there is insufficient evidence for a relation between shift work exposure and body weight change when confounders are taken into account.
The results of this review have to be interpreted with caution because of the limited number of studies, the methodological quality of the studies, and the heterogeneity between the in- cluded studies. The small amount of eligible studies was rather surprising regarding the large amount of shift work related literature that has been published. A majority of the articles had to be excluded because, as a result of different study objectives, the relevant outcome data was not presented. The available body weight data of their studies might have altered our results though. Eventually, only five out of eight included studies could be used to investigate the adjusted relation between shift work exposure and body weight change. If the three remaining studies would have made adjustments for potentially relevant confounders, other conclusions might have been drawn for this relation.
One of those potential confounders is physical activity during leisure time. This kind of physical fitness is an important factor influencing body weight gain, it might improve individual tolerance to shift work (19,45,46) and has a positive effect on sleep amount and quality, leading to a reduction in metabolic disturbances (19,47). Shift work literature generally states that employees involved in shift work become physically less active, have less time to participate in (organised, social) sporting activities and find it difficult to stay physically fit (46).
Consequently, if this is true, than physical activity would be an intermediate factor rather than a confounder. In this review however, none of the included studies analysed physical activity as an intermediate factor. Moreover, they did not analyse physical activity as an individual confounder, but adjusted their results together with the other confounders (37-40). Hence, another analysis of the role of physical activity may give more insight in the relation between shift work and body weight gain.
Of course, the lack of thorough analysis mentioned above holds true for other factors as well.
Factors as age and gender should be analysed for possible effect modifying effects. Moreover, longitudinal studies are most suitable to examine if variables as smoking, energy intake, and physical activity during work, which are routinely included in shift work studies, should be treated as intermediate factors.
4 Further, to summarize the results of the studies included, a levels of evidence synthesis,
which is often used for reviews concerning studies with a longitudinal design, was applied.
Due to this synthesis, the low quality studies were disregarded when multiple high quality studies were available. Because of this, the quality assessment and the chosen distinction between high and low quality studies are very influential. In the present study similar weight was given to all quality items. This resulted in the situation that a study could be considered as high quality although shift work exposure was vaguely assessed, no confounders had been used, or weight change was retrospectively self-reported. It is arguable if this kind of weight allocation is correct.
Another important issue in this review is the fact that when comparing the scores on the quality criteria of the different studies (Table 2), profound reasons for possible selection bias can be observed. Only four out of eight articles had a clear description of their in- and exclu- sion criteria and yet three studies had a non-selective non-response or a participation rate of at least 80%. Additionally, during follow-up, in some studies the response rate did not reach 80% or there was no response rate presented at all. Selection into shift work may be associ- ated with better adaptability and fewer adverse health effects. This leaves the most suitable, healthy people in the job (healthy worker effect), thereby underestimating the real effect of shift work exposure (5,45).
This review studied the effect of shift work which included night work because working dur- ing nights alters the circadian rhythm, which could lead to adverse health effects via several pathways (1,5-9). However, two studies were included which partly used participants who were not exposed to the working hours defined as being night work (12am-6am). Geliebter et al. (43) used fixed evening workers (27 out of the 49 late shift participants) and 27.5% of the participants in the study of Morikawa et al. (38) were on a two shift system, which normally does not involve night shifts. It is arguable if these numbers have over- or underestimated the association found in these studies, but they at least blurred the association this review was looking for.
Lack of a complete description of exposure might have underestimated the results as well.
Only three studies presented information about shift work exposure before the beginning of the study (38,39,41). Further, during the follow-up period, the exact exposure was not entirely clear either. Although it was adequately described in the retrospective (43,44) and the short- term follow-up studies (37,40,41), the other studies only assessed the shift work exposure at baseline and at the end of the follow-up, which implies that it cannot be ruled out that the workers might have changed type of (shift) work several times during this period (38,39,42).
Moreover, large differences between studies in follow-up duration were observed. Only two studies reported sufficient follow-up periods, 10 (38) and 14 years (37) respectively, to expect
real effects of the shift work exposure. Although Suwazono et al. (37) and Van Amelsvoort et al. (40) found significant differences in body weight gain between groups after one year, larger effects and effects in a similar direction might have been found if all studies would have used longer periods of follow-up.
Findings compared to other reviews
Earlier reviews that have addressed the problem of shift work and body weight change did not come to the same conclusion. In a narrative review published in 2003, Knutsson (6) stated that the evidence for the impact of shift work on body weight is inconsistent, and the evidence for a relation with metabolic factors of diabetes is inconclusive. In a recent review, Antunes et al.
(48) merely stated that shift work plays a role in increasing BMI.
In these reviews, both longitudinal and cross-sectional data were used to explain the find- ings, no differentiation was made between crude and adjusted associations, and the methods used were non-systematic. Moreover, they did not perform a methodological quality assess- ment and did not use a levels of evidence synthesis.
Explanation of the findings
Our findings are in accordance with shift work literature, which claims that, shift work expo- sure can lead to unhealthy behaviour and to subsequent disturbances in gastrointestinal and psychophysiological functioning causing body weight gain and obesity (9,26,38).
The review presented here found insufficient evidence for an adjusted relation between shift work exposure and body weight gain. However, two Japanese studies adjusted for health behaviour and still found a relation between shift work and body weight gain (37,38).
Although they did not measure food intake, the studies explain their results by claiming that the timing and amount of food intake is involved. However, two recent reviews state that total energy intake seems not to be affected by shift work, although meal frequency is irregular and reduced, and that high-energy snacking seems to be increased (19,49). Therefore, it seems that food intake itself cannot fully explain possible body weight gain as a result of shift work. It has become clear though, that irregular meal pattern and time of day intake could contribute to negative consequences in metabolism (20) because of the relation between circadian rhythm and food intake (19).
During daytime, when individuals normally eat, the human body promotes glucose metabo- lism and fat storage, while during the night, glucose sparing and fat metabo lism is promoted.
As a result, shift workers show a lowered glucose and lipid tolerance following the change from day to night working (50). Other studies reported about increased leptin and blood lipid
